Abstract
We analyzed the effect of development mode (direct vs. pelagic development) on the spatial structure of a megabenthic gastropod assemblage at a regional scale. The research was carried out along the bathymetric, saline and thermal gradients generated by the interaction between the shelf topography, the freshwater discharge of Rio de la Plata and the presence of oceanic water masses. Direct developers dominated in terms of number of species in the entire area and when the inner and outer shelves were considered separately. Species with pelagic development were not observed to be ecologically successful (i.e. have higher abundances) on the inner shelf, which suggests that high environmental variability may not necessarily favour this strategy. No patterns were found in the relative abundance of species in each developmental mode between the inner (i.e. < 50 m) and outer shelves. However, richness patterns of the two groups were differentially affected by environmental conditions. Species richness for direct developers was affected by mean annual temperature, bathymetry and longitude while pelagic developers showed evident trends in relation to temperature range and latitude. Further studies are necessary to derive general predictions concerning the relative advantages of each developmental type in relation to these gradients in ecological time scales and local or regional spatial scales.
Highlights
Developmental modes in marine invertebrates may be classified according to the place of development and trophic mode
Direct developers dominated in terms of number of species in the entire area and when the inner and outer shelves were considered separately
A similar phenomenon was described by Rass (1935, 1986) for marine fishes, who hypothesized that at lower temperatures species-specific egg size tended to increase due to temperature effects on the stages of oogenesis. Another key factor is the disturbance regime, which is invoked to explain either the relative rarity of species with planktonic larvae at polar regions (Palma et al, 2006; Potthoff et al, 2006) or the ecological dominance of broadcasters within Antarctic shallow water invertebrates (Poulin et al, 2002). While the latter phenomenon may be the result of processes operating at ecological timescales that are associated with the advantage of having pelagic larvae under highly disturbed conditions, the former may represent the outcome of species-level selection that occurs over geological and evolutionary time scales (Poulin et al, 2002)
Summary
Developmental modes in marine invertebrates may be classified according to the place of development and trophic mode. A similar phenomenon (an inverse relationship between egg size and water temperature) was described by Rass (1935, 1986) for marine fishes, who hypothesized that at lower temperatures species-specific egg size tended to increase due to temperature effects on the stages of oogenesis Another key factor is the disturbance regime, which is invoked to explain either the relative rarity of species with planktonic larvae at polar regions (Palma et al, 2006; Potthoff et al, 2006) or the ecological dominance of broadcasters within Antarctic shallow water invertebrates (Poulin et al, 2002). This highlights the need to account explicitly for the temporal and spatial scales of the phenomenon under study
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